Offset electrode tft structure

ABSTRACT

The present invention generally relates to an offset electrode TFT and a method of its manufacture. The offset electrode TFT is a TFT in which one electrode, either the source or the drain, surrounds the other electrode. The gate electrode continues to be below both the source and the drain electrodes. By redesigning the TFT, less voltage is necessary to transfer the voltage from the source to the drain electrode as compared to traditional bottom gate TFTs or top gate TFTs. The offset electrode TFT structure is applicable not only to silicon based TFTs, but also to transparent TFTs that include metal oxides such as zinc oxide or IGZO and metal oxynitrides such as ZnON.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/289,033 (APPM/15544), filed Nov. 4, 2011, which claims benefit ofU.S. Provisional Patent Application Ser. No. 61/448,429 (APPM/15544L),filed Mar. 2, 2011, which is herein incorporated by reference.

GOVERNMENT RIGHTS IN THIS INVENTION

This Invention was made with Government support under Agreement No.DAAD19-02-3-0001 awarded by ARL. The Government has certain rights inthis Invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to a thin filmtransistor (TFT) and a method of its manufacture.

2. Description of the Related Art

Current interest in TFT arrays is particularly high because thesedevices may be used in liquid crystal active matrix displays (LCDs) ofthe kind often employee for computer and television flat panels. TheLCDs may also contain light emitting diodes (LEDs), such as organiclight emitting diodes (OLEDs) for back lighting. The LEDs and OLEDsrequire TFTs for addressing the activity of the displays.

Bottom gate TFTs made with amorphous silicon have been utilized for theflat panel display industry for many years. Unfortunately, the on andoff-current driven through source and drain electrodes of the TFT islimited by its channel material as well as the channel width and length.Additionally, the turn-on gate voltage, or a threshold voltage, under avoltage between the source and drain electrodes, is determined by theaccumulation of the carrier in the active channel area of thesemiconductor active layer which could change as the change of thecharge in the channel material, dielectric material as well asinterfaces between the materials after bias temperature stress orcurrent temperature stress.

Therefore, there is a need in the art for a TFT that utilizes anadditional source-drain current controlling layer beneath the source ordrain electrode or both to miminize the off-current when the gatevoltage sets at a turn-off voltage. Because the additional control ofthe source-drain current control, the channel length between thesource-drain can be significantly reduced for high on-current when theTFT turns on, keeping the off-current low when the TFT turns off. Inaddition, the TFT design keeps the turn-on voltage, or threshold gatevoltage, unchanged after operations under different conditions.

SUMMARY OF THE INVENTION

The present invention generally relates to an offset electrode TFT and amethod of its manufacture. The offset electrode TFT is a TFT in whichone electrode, either the source or the drain, surrounds the otherelectrode. The gate electrode continues to be below both the source andthe drain electrodes. By redesigning the TFT, less voltage is necessaryto transfer the voltage from the source to the drain electrode ascompared to traditional bottom gate TFTs or top gate TFTs. The offsetelectrode TFT structure is applicable not only to silicon based TFTs,but also to transparent TFTs that include metal oxides such as zincoxide or IGZO and metal oxynitrides such as ZnON.

In one embodiment, a TFT is disclosed. The TFT comprises a gateelectrode disposed above a substrate; a gate dielectric layer disposedover the gate electrode; a channel semiconductor layer disposed over thegate dielectric layer; a first electrode disposed over the channelsemiconductor layer and at least partially defining a via; and a secondelectrode disposed over the channel semiconductor layer, within the viaand extending over at least a portion of the first electrode.

In another embodiment, a TFT includes a gate electrode, a sourceelectrode disposed over the gate electrode and a drain electrodedisposed over the source electrode with dielectric or semiconductorlayer in between.

In another embodiment, a TFT fabrication method is disclosed. The methodincludes depositing a gate dielectric layer over a gate electrode,depositing a channel semiconductor layer over the gate dielectric layer,depositing a source electrode over the channel semiconductor layer anddepositing a first source dielectric layer over the source electrode.The method also includes depositing a second source dielectric layerover the first source dielectric layer and removing at least a portionof the source electrode, the first source dielectric layer and thesecond source dielectric layer to form a via that is bound by edges ofthe source electrode, the first source dielectric layer and the secondsource dielectric layer and expose at least a portion of the controlsemiconductor layer. The method also includes depositing a spacer layerover at least a portion of the control semiconductor layer and the edgesof the source electrode, the first source dielectric layer and thesecond source dielectric layer. The method additionally includesdepositing a first control semiconductor layer over the exposed channelsemiconductor layer, depositing a second control semiconductor layerover the first control semiconductor layer and depositing a drainelectrode over the second control semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIGS. 1A-1V are schematic cross-sectional and top views of an offset TFT100 at various stages of production. FIGS. 1A, 1C, 1E, 1G, 1I, 1K, 1M,1O, 1Q, 1S and 1U are cross-sectional views while FIGS. 1B, 1D, 1F, 1H,1J, 1L, 1N, 1P, 1R, 1T and 1V are top views.

FIGS. 2A and 2B are schematic cross-sectional and top views of an offsetTFT 200 according to another embodiment.

FIGS. 3A and 3B are schematic cross-sectional and top views of an offsetTFT 300 according to another embodiment.

FIGS. 4A and 4B are schematic cross-sectional and top views of an offsetTFT 400 according to another embodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

The present invention generally relates to an offset electrode TFT and amethod of its manufacture. The offset electrode TFT is a TFT in whichone electrode, either the source or the drain, surrounds the otherelectrode. The gate electrode continues to be below both the source andthe drain electrodes. By redesigning the TFT, less off-current from thesource to the drain electrode is achieved when the gate voltage sets atan off voltage as compared to traditional bottom gate TFTs or top gateTFTs. It will also make the threshold voltage of the gate electrode, atwhich TFT starts to turns on or off, less sensitive of the change ofactive layer, dielectric layer and their interfaces. The offsetelectrode TFT structure is applicable not only to silicon based TFTs,but also to transparent TFTs that include metal oxides such as zincoxide or IGZO and metal oxynitrides such as ZnON.

Current TFTs have a gate electrode, a gate dielectric layer, asemiconductor channel, a source electrode and a drain electrode. Avertical TFT structure was presented in a paper that published in 2005.The vertical TFT has a supercapacitor, a thin rough electrode, asemiconductor, and a drain electrode. In the vertical TFT, thesupercapacitor is made of LiF. In the invention discussed herein, thesupercapacitor comprises metal oxides or metal oxynitride or acombination with thin film semiconductors such as a-Si or possible witha dielectric layer, but not LiF specifically, or other electrolyticmaterials. Therefore, the capacitor will be made using CVD or PVDprocesses commonly utilized in semiconductor fabrication and equipment.In addition, the electrodes will be made with oxide or oxynitride basedcomposite materials, instead of thin rough electrodes. The inventionincludes the materials to make the TFT, the materials for the capacitor,the materials for the electrodes and the material configuration; thefilm stack and process; and a solution for high off current. Theinvention is a new way to make a TFT and could provide unique featuresto address the stability issues with are encountered in current TFTstructures.

In the state of the art TFT, when gate voltage is set at an off voltage,certain level current still drives through TFT source and drainelectrodes under a source-drain voltage, depending on its channelmaterial as well as the channel width and length. Because of possiblehigh off-current, the short channel device is not practical although itcan provide high on-current at a low voltage. In addition, its turn-ongate voltage, or threshold voltage, under a voltage between the sourceand drain electrodes is determined by the accumulation of the carrier inthe channel layer, which could change with the change of the electricalcharge in the channel material, dielectric materials, and theirinterfaces composed of the TFT after bias temperature stress or currenttemperature stress. The TFT proposed herein has a gate, a gatedielectric, a channel semiconductor layer, and one metal electrode indirect contact with the semiconductor layer serves as source, which issimilar as the state-of-the-art TFT. However, the TFT also has a spacerand a control semiconductor on the top of the channel semiconductor butbeneath the drain electrodes. The new TFTs proposed herein will providea higher current than the state-of-the-art TFTs since the actual channellength will be much shorter when the device is turned on. Since thesource-drain current is controlled by the control semiconductorlayer(s), off-current of the short-channel device can be minimized. Inaddition, the charge trapping at the interfaces in the channel regionand bulk (the channel layer under the spacer) becomes less critical forthe turn on voltage for the device, since the turn-on voltage the devicewill be also determined by the accumulation in the control region whichhas channel semiconductor, control semiconductor and drain electrode.Therefore, threshold gate voltage shift caused by a change of charge ininterface and bulk layers should be significantly minimized. Theinvention includes: a TFT configuration comprising the TFT composed of agate, gate dielectric, channel semiconductor, source electrode, controlsemiconductor or semiconductors, drain electrode; a TFT configurationcomprising the TFT has a channeling region and one or more controllingregions. The channel semiconductor and the control semiconductormaterial can be different or the same. Additionally, an additionalbarrier layer may be present in between the semiconductor layers. Forexample, metal oxide or metal oxynitride is used as channelsemiconductor, and amorphous Si is used as control semiconductor.Alternatively, the metal oxynitride may be used as both the channelsemiconductor and control semiconductor; however the carrierconcentration or the band gap will be different or a thin SiO or SiNlayer will be present in between the semiconductor layers. The spacer ismade of SiO or SiN, or other dielectric materials. The spacer is madefrom a conformal deposition and etch back process. The controllingregion for on-off can result from a barrier between the channelsemiconductor and control semiconductor, or a barrier created inside thecontrol semiconductor material, or a barrier between the source metaland the channel semiconductor or between the drain metal and the controlsemiconductor. The barrier can result from the Fermi level difference,work function difference, band gap difference, or others film propertydifference.

FIGS. 1A-1V are schematic cross-sectional and top views of a TFT 100 atvarious stages of production. FIGS. 1A, 1C, 1E, 1G, 1I, 1K, 1M, 1O, 1Q,1S and 1U are cross-sectional views while FIGS. 1B, 1D, 1F, 1H, 1J, 1L,1N, 1P, 1R, 1T and 1V are top views. As shown in FIGS. 1A and 1B, a gateelectrode 104 is formed over a substrate 102. Suitable materials thatmay be utilized for the substrate 102 include, but may not be limitedto, silicon, germanium, silicon-germanium, soda lime glass, glass,semiconductor, plastic, steel or stainless steel substrates. Suitablematerials that may be utilized for the gate electrode 104 include, butmay not be limited to, chromium, copper, aluminum, tantalum, titanium,molybdenum, and combinations thereof, or conductive transparent oxides(TCO) such as ITO (indium tin oxide) or ZnO:F commonly used astransparent electrodes. The gate electrode 104 may be deposited bysuitable deposition techniques such as physical vapor deposition (PVD),MOCVD, spin-on process, and printing processes. If necessary, the gateelectrode 104 may be patterned using an etching process.

Over the gate electrode 104, a gate dielectric layer 106 may bedeposited as shown in FIGS. 1C and 1D. Suitable materials that may beused for the gate dielectric layer 106 include silicon dioxide, siliconoxynitride, silicon nitride, aluminum oxide or combinations thereof. Thegate dielectric layer 106 may be deposited by suitable depositiontechniques including plasma enhanced chemical vapor deposition (PECVD).

A channel semiconductor layer 108 is then formed over the gatedielectric layer 106 as shown in FIGS. 1E and 1F. The channelsemiconductor layer 108 may comprise traditional semiconductor materialused in TFTs such as amorphous silicon or polysilicon. Additionally,next generation semiconductor materials are also contemplated such asmetal oxides including zinc oxide (ZnO) and indium-gallium-zinc oxide(IGZO) as well as oxynitrides such as indium-gallium-zinc-oxynitride(IGZON) and zinc oxynitride (ZnON). Other materials that arecontemplated include ZnO_(x)N_(y), SnO_(x)N_(y), InO_(x)N_(y),CdO_(x)N_(y), GaO_(x)N_(y), ZnSnO_(x)N_(y), ZnInO_(x)N_(y),ZnCdO_(x)N_(y), ZnGaO_(x)N_(y), SnInO_(x)N_(y), SnCdO_(x)N_(y),SnGaO_(x)N_(y), InCdO_(x)N_(y), InGaO_(x)N_(y), CdGaO_(x)N_(y),ZnSnInO_(x)N_(y), ZnSnCdO_(x)N_(y), ZnSnGaO_(x)N_(y), ZnInCdO_(x)N_(y),ZnInGaO_(x)N_(y), ZnCdGaO_(x)N_(y), SnInCdO_(x)N_(y), SnInGaO_(x)N_(y),SnCdGaO_(x)N_(y), InCdGaO_(x)N_(y), ZnSnInCdO_(x)N_(y),ZnSnInGaO_(x)N_(y), ZnInCdGaO_(x)N_(y), and SnInCdGaO_(x)N_(y). Each ofthe aforementioned semiconductor films may be doped by a dopant such asAl, Sn, Ga, Ca, Si, Ti, Cu, Ge, In, Ni, Mn, Cr, V, Mg, Si_(x)O_(y),Si_(x)N_(y), Al_(x)O_(y), and SiC. The channel semiconductor layer 108may be deposited by a suitable deposition method such as PVD, PECVD,chemical vapor deposition (CVD), or atomic layer deposition (ALD),spin-on or printing processes. The channel semiconductor layer 108permits the current to flow between the source and drain electrodes oncethe gate electrode 104 is biased. The channel semiconductor layer 108may be patterned, if desired, by a wet etching process.

As shown in FIGS. 1G and 1H, over the channel semiconductor layer 108, aconductive layer 110 is deposited that will eventually form the firstelectrode which can be either the source or the drain electrode,depending upon the electrical connection. Hereinafter, it is to beunderstood that reference to the first electrode includes either thesource electrode or the drain electrode. Suitable materials for theconductive layer 110 include chromium, copper, aluminum, tantalum,titanium, molybdenum, and combinations thereof, or TCOs mentioned above.The conductive layer 110 may be deposited by suitable depositiontechniques, such as PVD. The conductive layer could be patterned throughetching or a printing process as the first electrode, or not patterned,or partially patterned. The following gives the example that the firstelectrode is not patterned or partially patterned, (i.e., outside thefirst electrode is defined but the control portion of the firstelectrode is not defined or formed yet).

A first dielectric layer 112 is then deposited over the conductive layer110 as shown in FIGS. 1I and 1J. Suitable materials that may be used forthe first dielectric layer 112 include silicon dioxide, siliconoxynitride, silicon nitride, or combinations thereof. The firstdielectric layer 112 may be deposited by suitable deposition techniques,including PECVD. A second dielectric layer 114 is then deposited overthe first dielectric layer 112 as shown in FIGS. 1K and 1L. Suitablematerials that may be used for the second dielectric layer 114 includesilicon dioxide, silicon oxynitride, silicon nitride, or combinationsthereof. The second dielectric layer 114 may be deposited by suitabledeposition techniques, including PECVD. In one embodiment, the firstdielectric layer 112 and the second dielectric layer 114 compriseseparate, distinct layers that comprise different materials. In anotherembodiment, the first dielectric layer 112 and the second dielectriclayer 114 may comprise a single layer.

The second dielectric layer 114, the first dielectric layer 112 and theconductive layer 110 are then patterned as shown in FIGS. 1M and 1N tocreate the first electrode 116, patterned first dielectric layer 118 andpatterned second dielectric layer 120. As shown in FIG. 1N, a via 119 ispresent such that the first electrode 116, the patterned firstdielectric layer 118 and the patterned second dielectric layer 120 boundthe via 119. In the embodiment shown in FIG. 1N, the first electrode 116has a slot 121 therein such that the first electrode 116 does notcompletely encircle the exposed channel semiconductor layer 108, butdoes at least partially encircle the exposed channel semiconductor layer108. However, it is to be understood that the first electrode 116 couldcompletely encircle the channel semiconductor layer 108 or with a largeopening cross several sides. After the patterning, a portion of thechannel semiconductor layer 108 is exposed. While the patterning isshown as occurring after the second dielectric layer 114, the firstdielectric layer 112 and the conductive layer 110 have all beendeposited, it is contemplated that the patterning may occur after thedeposition of the second dielectric layer 114, the deposition of thefirst dielectric layer 112 and the deposition of conductive layer 110.Additionally, it is contemplated that the patterning may occur after thedeposition of the conductive layer 110 and then again after thecollective deposition of the first dielectric layer 112 and the seconddielectric layer 114. The patterning may occur by forming a mask overthe uppermost layer (i.e., the second dielectric layer 114 in FIGS. 1Kand 1L) and then etching the exposed surfaces. Different etchingconditions may be necessary for each layer etched.

A spacer layer is then deposited over the exposed surfaces andselectively removed so that a spacer 122 remains along the edges of thefirst electrode 116, the patterned first dielectric layer 118 and thepatterned second dielectric layer 120. The spacer 122 is also present onthe now exposed channel semiconductor layer 108. However, as shown inFIGS. 1O and 1P, the spacer 122 does not cover the entire channelsemiconductor layer 108 as the spacer layer has been removed fromselected portions of the channel semiconductor layer 108. Thus, aportion of the channel semiconductor layer 108 remains exposed after theformation of the spacer 122. Suitable materials that may be used for thespacer 122 include silicon dioxide, silicon oxynitride, silicon nitride,or combinations thereof. The spacer 122 may be deposited by suitableconformal deposition techniques including PECVD, CVD and ALD. After theconformal deposition process, a spacer etch or photo-resist patterningplus an etch process occurs to form the spacer 122. After the spacerformation, the second source dielectric layer may or may not exist. Ifthe second dielectric layer is removed during the spacer etch, thethickness of the first source dielectric may or may not be reduced. Thedielectric layers 118, 120 and spacer 122 can insulate the first andsecond electrodes 116, 128 from each other. The spacer layer 122 can beused for selective etching. For example, the spacer 122 can be formed bya conformal deposition process formed over the second dielectric layer120 and the sidewalls that surround the channel semiconductor layer 108.The second dielectric layer 120 is harder to etch than the spacer 122,the spacer 122 can be over etched without any fear of losing dielectricmaterial over the electrode 116 during the spacer etch process.

A first control semiconductor layer 124 is then formed over the exposedchannel semiconductor layer 108, the spacer 122 and the patterned seconddielectric layer 120. The first control semiconductor layer 124 isformed by blanket depositing a layer and then etching the layer to leavethe resulting structure shown in FIGS. 1Q and 1R. The first controlsemiconductor layer 124 may comprise traditional semiconductor materialused in TFTs such as amorphous silicon or polysilicon. Additionally,next generation semiconductor materials are also contemplated such asIGZO and ZnON. Other materials are contemplated such as boron doped orphosphorous doped, or no doped amorphous silicon and its combinationwith these or with others. Additionally materials that are contemplatedinclude ZnO_(x)N_(y), SnO_(x)N_(y), InO_(x)N_(y), CdO_(x)N_(y),GaO_(x)N_(y), ZnSnO_(x)N_(y), ZnInO_(x)N_(y), ZnCdO_(x)N_(y),ZnGaO_(x)N_(y), SnInO_(x)N_(y), SnCdO_(x)N_(y), SnGaO_(x)N_(y),InCdO_(x)N_(y), InGaO_(x)N_(y), CdGaO_(x)N_(y), ZnSnInO_(x)N_(y),ZnSnCdO_(x)N_(y), ZnSnGaO_(x)N_(y), ZnInCdO_(x)N_(y), ZnInGaO_(x)N_(y),ZnCdGaO_(x)N_(y), SnInCdO_(x)N_(y), SnInGaO_(x)N_(y), SnCdGaO_(x)N_(y),InCdGaO_(x)N_(y), ZnSnInCdO_(x)N_(y), ZnSnInGaO_(x)N_(y),ZnInCdGaO_(x)N_(y), and SnInCdGaO_(x)N_(y). Each of the aforementionedsemiconductor films may be doped by a dopant such as Al, Sn, Ga, Ca, Si,Ti, Cu, Ge, In, Ni, Mn, Cr, V, Mg, Si_(x)N_(y), Al_(x)O_(y), and SiC.The first control semiconductor layer 124 may be deposited by suitabledeposition methods such as PVD, PECVD, CVD, or ALD. The first controlsemiconductor layer 124 is disposed over the source electrode 116, thepatterned first source dielectric layer 118 and the patterned secondsource dielectric layer 120.

The purpose of a control semiconductor layer is to create an additionalbarrier for electrons to flow easily in one direction, yet difficult forelectrons to flow in the opposite direction, such as a diode orrectifying effect. The control semiconductor layer lets electrons flowonly under certain source-drain voltage differences. A controllingsemiconductor layer has a different composition than the adjacentchannel semiconductor layer. Without a controlling semiconductor layer,the barrier could still be created based upon the difference of workfunctions, surface charge trapping or even surface defects that arecreated by intentionally creating defects in the semiconductor layer.

Over the first control semiconductor layer 124, a second controlsemiconductor layer 126 is formed. The second control semiconductorlayer 126 is formed by blanket depositing a layer and then etching thelayer to leave the resulting structure shown in FIGS. 1S and 1T. Thesecond control semiconductor layer 126 may comprise traditionalsemiconductor material used in TFTs such as amorphous silicon orpolysilicon. Additionally, next generation semiconductor materials arealso contemplated such as IGZO and ZnON. Other materials that arecontemplated include ZnO_(x)N_(y), SnO_(x)N_(y), InO_(x)N_(y),CdO_(x)N_(y), GaO_(x)N_(y), ZnSnO_(x)N_(y), ZnInO_(x)N_(y),ZnCdO_(x)N_(y), ZnGaO_(x)N_(y), SnInO_(x)N_(y), SnCdO_(x)N_(y),SnGaO_(x)N_(y), InCdO_(x)N_(y), InGaO_(x)N_(y), CdGaO_(x)N_(y),ZnSnInO_(x)N_(y), ZnSnCdO_(x)N_(y), ZnSnGaO_(x)N_(y), ZnInCdO_(x)N_(y),ZnInGaO_(x)N_(y), ZnCdGaO_(x)N_(y), SnInCdO_(x)N_(y), SnInGaO_(x)N_(y),SnCdGaO_(x)N_(y), InCdGaO_(x)N_(y), ZnSnInCdO_(x)N_(y),ZnSnInGaO_(x)N_(y), ZnInCdGaO_(x)N_(y), and SnInCdGaO_(x)N_(y). Each ofthe aforementioned semiconductor films may be doped by a dopant such asAl, Sn, Ga, Ca, Si, Ti, Cu, Ge, In, Ni, Mn, Cr, V, Mg, Si_(x)N_(y),Al_(x)O_(y), and SiC. Other materials are contemplated such as borondoped or phosphorous doped, or no doped amorphous silicon and itscombination with these or with others. The second control semiconductorlayer 126 may be deposited by suitable deposition methods, such as PVD,PECVD, CVD, or ALD. The second control semiconductor layer 126 isdisposed over the first control semiconductor layer 124, the sourceelectrode 116, the patterned first source dielectric layer 118 and thepatterned second source dielectric layer 120. Dielectric layer 120 couldbe removed partially or fully during formation of the dielectric spacer122. The second control semiconductor layer 126 is used for tuning thebarrier.

Finally, over the second control semiconductor layer 126, the secondelectrode 128 is formed as shown in FIGS. 1U and 1V. The secondelectrode 128 is formed by blanket depositing a conductive material andthen etching the conductive material to form the final structure of thesecond electrode 128. Suitable materials for the second electrode 128include chromium, copper, aluminum, tantalum, titanium, molybdenum, andcombinations thereof, or TCOs mentioned above. The second electrode 128may be deposited by suitable deposition techniques, such as PVD. Asshown in FIGS. 1U and 1V, the first electrode 116, while disposed belowthe second electrode 128, in essence surrounds the second electrode 128because the first electrode 116 forms at least a portion of the via 119in which the second electrode 128 is deposited. However, it should benoted that a portion of the second electrode 128 is disposed over thefirst electrode 116, the patterned first dielectric layer 118, thepatterned source dielectric layer 120, the first control semiconductorlayer 124 and the second control semiconductor layer 126.

Electrode 128 covers the channel semiconductor layer 108 as shown inFIGS. 1U and 1V to protect the channel semiconductor layer 108 fromlight. If the channel semiconductor layer 108 is exposed to light, thenthe channel semiconductor layer 108 becomes conductive.

The channel semiconductor layer 108 and the control semiconductor layers124, 126 may comprise different materials. For example, the channelsemiconductor layer 108 may comprise a metal oxide or a metal oxynitridewhile the control semiconductor layers 124, 126 may comprise amorphoussilicon. In one embodiment, the channel semiconductor layer 108 and thecontrol semiconductor layers 124, 126 comprise the same material. Forexample, the control semiconductor layers 124, 126 and the channelsemiconductor layer 108 may comprise a metal oxynitride. It iscontemplated that a barrier layer (not shown) may be present between thechannel semiconductor layer 108 and the first control semiconductorlayer 124. Suitable materials for the barrier layer include siliconoxide or silicon nitride, or not doped or doped amorphous silicon. Thecontrolling region of the on-off for the TFT 100 results from a barrierbetween the channel semiconductor layer 108 and the first controlsemiconductor layer 124, a barrier created inside the controlsemiconductor material or a barrier between the source electrode 116 andthe first channel semiconductor layer 124, or a barrier between thedrain electrode 128 and the second control semiconductor layer 126. Thebarrier created inside the control semiconductor material can resultfrom the Fermi level difference, work function difference, band gapdifference, or other film property differences.

FIGS. 2A and 2B are schematic cross-sectional and top views of an offsetTFT 200 according to another embodiment. The TFT 200 includes asubstrate 202, gate electrode 204, gate dielectric layer 206 and channelsemiconductor layer 208. However, there is both a first controlsemiconductor layer 210, 222 and a second control semiconductor layer212, 224 that are present adjacent to each electrode 214, 226. Eitherelectrode 214, 226 may function as the source electrode while the otherelectrode 214, 226 functions as the drain electrode. The first electrode214 could completely encircle the channel semiconductor layer 108 orwith a large opening cross several sides not shown. Both the firstcontrol semiconductor layer 210, 222 and the second controlsemiconductor layer 212, 224 are below each of the first and secondelectrodes 214, 226. Additionally, multiple dielectric layers 216, 218and a spacer 220 are present. Dielectric layer 218 could be removedpartially or fully during formation of the dielectric spacer 222. Thus,in the embodiment shown in FIGS. 2A and 2B, control semiconductor layersare present at both the first and second electrodes 214, 226 whereas inthe embodiment shown in FIGS. 1A-1V, the control semiconductor layersare present at only one electrode. The control semiconductor layers 210,212, 222, 224 are spaced apart by spacer 220.

FIGS. 3A and 3B are schematic cross-sectional and top views of an offsetTFT 300 according to another embodiment. The TFT 300 includes asubstrate 302, gate electrode 304, gate dielectric layer 306 and channelsemiconductor layer 308. Over the channel semiconductor layer 308, afirst control semiconductor layer 310 is present. Additionally, there isboth a second control semiconductor layer 312, 324 and a third controlsemiconductor layer 314, 326 that are present. Both the second controlsemiconductor layer 312, 324 and the third control semiconductor layer314, 326 are below each of the first and second electrodes 316, 328. Thefirst electrode 316 could completely encircle the channel semiconductorlayer 108 or with a large opening cross several sides not shown.Additionally, multiple dielectric layers 318, 320 and a spacer 322 arepresent. Dielectric layer 320 could be removed partially or fully duringformation of the dielectric spacer 322. Thus, in the embodiment shown inFIGS. 3A and 3B, control semiconductor layers are present at both thefirst and second electrodes 316, 328, just as in FIGS. 2A and 2B.Control semiconductor layers 312, 314, 324, 324 are spaced apart byspacer 322, but there is an additional control semiconductor layer 310that spans across the entire channel semiconductor layer 308.

FIGS. 4A and 4B are schematic cross-sectional and top views of an offsetTFT 400 according to another embodiment. In TFT 400, no controlsemiconductor layers are present. Rather, the TFT 400 includes asubstrate 402, a gate electrode 404, a gate dielectric layer 406, achannel semiconductor layer 408 and first and second electrodes 412, 414spaced apart by a spacer 410. The first electrode 414 could completelyencircle the channel semiconductor layer 108 or with a large openingcross several sides not shown. Note that in each of FIGS. 2A, 2B, 3A,3B, 4A and 4B, between 20 percent and 100 percent of the active channel(i.e., channel semiconductor layer between the first and secondelectrodes) is covered. A metal oxide TFT is more stable when the activechannel is covered because the active channel is not exposed to lightwhich can make the active channel conductive rather than semiconductive.

The offset electrode TFT structure can be used widely on many electronicapplications such as OLED TV or other devices that require high currentand stable threshold voltages. The TFTs disclosed herein have a highercurrent as compared to the state of the art TFTs because the actualchannel length is much shorter when the device is turned on. Thedistance between the source and drain electrodes is reduced because ofthe location of the drain electrode relative to the source electrode.Additionally, the charge trapping at the interfaces in the channelregion (the channel layer under the spacer) becomes not-critical forturning on the device since turning on the device will be determined bythe accumulation in the control region which has channel semiconductormaterial, control semiconductor material and the drain electrode.

The TFTs described herein are beneficial for the next generation highdefinition displays. Due to one of the electrodes being over the top ofthe channel semiconductor layer, the TFT is smaller in size as comparedto the traditional bottom gate TFT. Because the TFT is smaller in size,more pixels may be contained within a smaller space (i.e., higherdensity of pixels). Additionally, because the TFT is offset whereby anelectrode covers the channel semiconductor layer, less energy is neededto illuminate the pixel. Finally, because one of the electrodes isformed over the channel semiconductor layer, metal oxides are notexposed to light which would otherwise render the metal oxidesconductive rather than semiconductive. Therefore, the TFTs describedherein are much more stable and reliable as compared to bottom gate andtop gate TFTs.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow. Additionally, the term “over” asused herein is meant to include objects that are both above and incontact with another object as well as objects that are above and not incontact with the other object.

1. A thin film transistor, comprising: a gate electrode disposed above asubstrate; a gate dielectric layer disposed over the gate electrode; achannel semiconductor layer disposed over the gate dielectric layer; afirst electrode disposed over the channel semiconductor layer and atleast partially defining a via, wherein the first electrode is either asource electrode or a drain electrode; and a second electrode disposedover the channel semiconductor layer, within the via and extending overat least a portion of the first electrode, wherein the second electrodeis either a source electrode or a drain electrode and is different fromthe first electrode.
 2. The thin film transistor of claim 1, wherein thechannel semiconductor layer comprises a metal oxide or a metaloxynitride.
 3. The thin film transistor of claim 2, wherein the channelsemiconductor layer comprises indium, gallium and zinc.
 4. The thin filmtransistor of claim 2, wherein the channel semiconductor layer compriseszinc oxynitride.
 5. The thin film transistor of claim 1, furthercomprising a spacer layer disposed over the first electrode and thechannel semiconductor layer, wherein the second electrode is disposed atleast partially over the spacer layer.
 6. The thin film transistor ofclaim 1, wherein at least a portion of the gate electrode, the firstelectrode and the second electrode are vertically aligned.
 7. A thinfilm transistor, comprising: a gate electrode; a first electrodedisposed over the gate electrode, wherein the first electrode is eithera source electrode or a drain electrode; and a second electrode disposedover the first electrode, wherein the second electrode is either asource electrode or a drain electrode and is different from the firstelectrode.
 8. The thin film transistor of claim 7, further comprising afirst semiconductor layer disposed between the gate electrode and thefirst electrode.
 9. The thin film transistor of claim 7, wherein thechannel semiconductor layer comprises indium, gallium and zinc.
 10. Thethin film transistor of claim 7, wherein the channel semiconductor layercomprises zinc oxynitride.
 11. The thin film transistor of claim 7,wherein at least a portion of the gate electrode, the first electrodeand the second electrode are vertically aligned.
 12. The thin filmtransistor of claim 7, wherein at least a portion of the first electrodeencircles at least a portion of a semiconductor layer disposed betweenthe first electrode and the second electrode.
 13. A thin film transistorfabrication method, comprising: depositing a gate dielectric layer overa gate electrode; forming a channel semiconductor layer over the gatedielectric layer; forming a first electrode over the channelsemiconductor layer, wherein the first electrode is either a sourceelectrode or a drain electrode; and forming a second electrode over thechannel semiconductor layer and over at least a portion of the firstelectrode, wherein the second electrode is either a source electrode ora drain electrode and is different from the first electrode.
 14. Themethod of claim 13, wherein the channel semiconductor layer comprisesindium, gallium and zinc.
 15. The method of claim 13, wherein thechannel semiconductor layer comprises zinc oxynitride.
 16. The method ofclaim 13, wherein the first electrode encircles at least a portion ofthe channel semiconductor layer.